Maybe I was lucky, but I had the same issue where I no longer had access to my old e-mail account and they sorted it out for me within a few hours (after a few e-mails back and forth).
It probably helped that I was able to provide them with a screenshot of the game in my Steam library as well as a list of all the games linked to my Rockstar account with dates of purchase. To find the dates of purchase on Steam I went to “account details” and “view purchase history”.
As far as I know, all Rimworld mods will work with Linux. You can either subscribe to them on the Steam Workshop (and enable them from the mod menu in-game) or download them manually and put them in the mods folder in the installation directory. I’ve played with modlists that had more than 100 mods in them and never had a Linux related issue.
To answer your other question, I dual booted Linux for a while, mainly because of privacy concerns, but switched to Linux full time around the time Windows 10 came out. The thing that gave me the final push was Windows 10 on my new laptop telling me it couldn’t open a zipped folder and I would need to pay for that feature! There was also a backup copy of W10 on a second drive that I didn’t know about which automatically overwrote Linux when I tried to install it.
I chose Mullvad because they don’t ask for any personal details and you can pay anonymously, which means that their service is privacy protecting by design. You don’t have to rely so much on trust.
Proton seems to be a large and rapidly expanding company which looks like it’s trying to be a more privacy respecting competitor to Google’s many services. While that’s not necessarily a bad thing, I prefer companies that value stability over rapid expansion. I also don’t like relying on a potential single point of failure for everything. I have a Proton e-mail account but I don’t use any of their other services because I don’t want everything in the same place.